Using intravenous infusion of deuterium-labeled nicotine, these investigators will study the kinetics and metabolism of nicotine in pairs of twins who are both smokers and non-smokers to examine genetic and environmental influences. They hope to determine if: (1) metabolic clearance has a significant genetic component; (2) if the fractional conversion of nicotine to cotinine has a higher heritable component than each of its separate components; (3) to address the relationship of smoking to metabolic and renal clearance and fractional conversion measures within monozygotic twins who are genetically identical.